CVE-2026-3590 — server
MEDIUMCVE-2026-3590 is a medium-severity (CVSS 6.5) CWE-367 vulnerability in github.com/mattermost/mattermost/server/v8. A fix is available for github.com/mattermost/mattermost/server/v8 — see the affected versions and patch details below.
Race Condition in Guest Magic Link Authentication Allows Token Reuse
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-3590.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
github.com/mattermost/mattermost/server/v8🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-serverReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
Mattermost versions 10.11.x <= 10.11.12, 11.5.x <= 11.5.0, 11.4.x <= 11.4.2, 11.3.x <= 11.3.2 fail to enforce atomic single-use consumption of guest magic link tokens, which allows an attacker with access to a valid magic link to establish multiple independent authenticated sessions via concurrent requests.. Mattermost Advisory ID: MMSA-2026-00624
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/mattermost/mattermost/server/v8 | ≥ 8.0.0-20250721062209-4952acea88ce&&< 8.0.0-20250723052842-4cb8d8940332 | 8.0.0-20250723052842-4cb8d8940332go get github.com/mattermost/mattermost/server/v8@v8.0.0-20250723052842-4cb8d8940332 |
| 🐹Go | github.com/mattermost/mattermost-server | ≥ 10.11.0-rc1&&< 10.11.13 | 10.11.13go get github.com/mattermost/mattermost-server@v10.11.13 |
| 🐹Go | github.com/mattermost/mattermost-server | ≥ 11.5.0-rc1&&< 11.5.0 | 11.5.0go get github.com/mattermost/mattermost-server@v11.5.0 |
| 🐹Go | github.com/mattermost/mattermost-server | ≥ 11.4.0-rc1&&< 11.4.3 | 11.4.3go get github.com/mattermost/mattermost-server@v11.4.3 |
| 🐹Go | github.com/mattermost/mattermost-server | ≥ 11.3.0-rc1&&< 11.3.3 | 11.3.3go get github.com/mattermost/mattermost-server@v11.3.3 |
Affected Products
mattermost servermattermostDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/mattermost/mattermost/server/v8, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/mattermost/mattermost/server/v8 to 8.0.0-20250723052842-4cb8d8940332 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-3590 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Frequently Asked Questions
Is CVE-2026-3590 in your dependencies?
Find it across Go, including transitive dependencies.